A tender unpublished letter from Nelson Mandela, written from his prison cell on Robben Island when he learned months late of the death of his friend and fellow anti-apartheid activist Michael Harmel, is expected to fetch up to £100,000 at auction.

Mandela wrote on 1 October 1974, 10 years into his life sentence, on both sides of a thin sheet of cheap paper covered in official prison stamps, to Harmel’s daughter Barbara and wife, Ray, four months after his death.

He said: “I have been thinking of you & Ray ever since I heard of the death of your darling Pa. I have no details whatsoever as to when & how he died, having received only the bare report of the fact from one who assumed that we already knew about it. It is not easy to accept that we will never see Mike again.”

Mandela had initially been doubtful about Harmel’s quality, he wrote. “I was convinced that he did not deserve the honour of being placed among the elite. It was some years later that I came to accept his simplicity as a virtue on which one could model his own life.”

By 1974 Mandela had been promoted from a grade D to a grade A prisoner, which meant that he was allowed more news from the outside world, but he slept on a stone floor in a cell from which he was released for stone-breaking during the week, and for one hour a day at weekends. He would finally be freed by FW de Klerk in 1990, to become his country’s first post-apartheid president in 1994.

In 1974 he must have feared he would never leave prison, but jokily promised to take his European “sisters” to a Xhosa feast. He ended: “It has been said that faith is like an oak tree, it grows steadily but once established it endures for centuries. Ever ridden a horse in your life, or seen a horse race? Hope is the horse on which you ride & travel to your destination to reach the winning post. My only fortune in life is to have friends who taught me these things, amongst whom was your beloved Pa.”

Harmel was a member of the Communist party, who worked briefly on the Daily Worker newspaper in London before returning to South Africa, where he became a trade unionist, activist, and the first editor in 1959 of the Communist African. In 1962, the same year Mandela was put on trial, he was placed under house arrest for five years, before being forced into exile. He died in Prague in June 1974, aged 58.

Ray Harmel was also a friend, who at Mandela’s request made the wedding dress for Winnie Mandela in 1958. The glamorous photographs of the couple after their marriage were taken at the Harmels’ home.

Matthew Haley, the head of books and manuscripts at Bonhams, where the letter will be auctioned on 12 September, called it “an amazing thing”.

“It is a beautiful letter, just the sort of letter one would want to receive on such a sad occasion, all the more special for the circumstances in which it was written,” he said. “It’s just so vivid, and that closing image of the horse race is so full of a sense of freedom and joy – so different from his actual situation.”

Although the letter will be auctioned in London, the document is still in South Africa, and may not be allowed to leave the country if bought by an overseas bidder. Haley hopes it may be acquired for a public collection in South Africa, where scores of Mandela’s prison letters are kept as national treasures.

• This article was amended on 30 August 2018. The letter will be auctioned on 12, not 14, September.